Conventionally, hermetic compressors have been widely known. For example, such hermetic compressors are provided in refrigerant circuits for refrigerators or air conditioners and are widely used for compressing refrigerants. In general, the hermetic compressors each include a casing as a sealed container and a compression mechanism accommodated within the casing. In the hermetic compressors, lubricant oil retained at the bottom of the casing is supplied to the compression mechanism and the like for lubrication.
In a hermetic compressor of this type, lubricant oil and a gas refrigerant coexist within the casing. For this reason, a considerable amount of refrigerant dissolves in the lubricant oil in a state when the external temperature is low or the like, which may lower the viscosity of the lubricant oil. When the compressor is driven under the condition that the viscosity thereof remains low, the lubricant oil of such low viscosity is supplied to the compression mechanism, which may cause lubrication malfunction and damage to the compressor.
In order to solve the above problem, countermeasures have been proposed in which the lubricant oil retained in the casing is heated to lower the amount of the refrigerant dissolving in the lubricant oil for the purpose of recovering the viscosity of the lubricant oil. For example, Japanese Laid Open Patent Application Publication No. 10-148405A discloses that an electric heater is wound around the casing and is conducted to heat the lubricant oil. Also, Japanese Laid Open Japanese Patent Application Publication No. 2000-130865A discloses that a refrigerant discharge path is provided along the outer circumference of the casing so as to heat the lubricant oil by utilizing the high temperature gas discharged from the compressor.
Problems that the Invention is to Solve
However, the above countermeasures in which the lubricant oil in the casing is heated avoid insufficiently the damage to the compressor caused by the low viscosity of the lubricant oil.
This problem will be explained in detail. In the above countermeasures, the electric heater or the high-temperature discharge gas heats the casing and indirectly heats the lubricant oil through the heated casing. The heat to be transmitted to the lubricant oil from the casing is transmitted gradually from the vicinity of the casing toward parts apart therefrom. Therefore, considerable time is required for increasing the temperature of the lubricant oil until the viscosity thereof is recovered sufficiently. For this reason, the viscosity of the lubricant oil remains low for a while even after the heating of lubricant oil starts, with a result that the lubrication malfunction for the moment remains and damage to the compressor may be invited.
The present invention has been made in view of the above disadvantages and has its object of surely avoiding lubrication malfunction caused due to lowered viscosity of the lubricant oil by dissolution of the refrigerant, and of enhancing reliability of the hermetic compressor.